Power amplifiers are often used in radiotelephones. Radiotelephone power amplifiers require sufficient gain, output power, and control range while maintaining efficiency of power supply current usage, small physical size, and weight. Power amplifiers typically include one or more cascaded amplifier stages. Each stage includes a transistor and circuits for biasing the transistor.
A silicon bipolar transistor or the silicon field effect transistor (FET) are most often used in radio frequency (RF) power amplifiers (PA). These devices usually require two positive-voltage power supplies: a main supply that feeds a large DC current into the collector or drain, and a biasing supply that controls the amount of DC current from the main supply. The biasing supply can also be manipulated to control the output power of the amplifier. It can be difficult to provide the gain and output power requirements of many radiotelephone power amplifiers which operate at the higher radio frequencies, such as 1.5 GHz and above, using transistors that are made with silicon. The difficulties are caused by gain and bandwidth limitations of the transistors.
Gallium arsenide, GaAs, Metal-Semiconductor field effect transistors, (MESFET) have higher gain and bandwidth due to higher electron mobility in gallium arsenide and are predominantly used at these frequencies. The higher gain and bandwidth of the MESFETs enables the design of power amplifiers with better user-perceived quality due to better efficiency of power supply current usage at lower radio frequencies such as 800 MHz. Like silicon transistors, GaAs MESFETs require two power supplies: a main supply connected to the drain and a biasing supply connected to the gate.
The GaAs MESFETs used in power amplifiers are usually n-channel depletion mode devices. In depletion mode devices, the gate threshold voltage, which is the voltage required for the device to begin conducting current from the drain to the grounded source, is negative, which means that the biasing supply must be capable of negative output voltages. GaAs MESFET based power amplifiers are operated optimally in class AB mode such that the gate voltage is set slightly above the negative threshold voltage. A manual or automatic biasing supply voltage adjustment is necessary for each amplifier because of variations of the gate threshold voltage from part to part and over temperature. Since at lower gate voltages, the gain and output power are reduced, the gate voltage can be varied to control the output power.
Additionally, it is necessary that the biasing power supply of a GaAs MESFET is always turned on before the main power supply is turned on. This is because while the biasing supply is turned off and the gate to source voltage is zero, the device is turned on and the drain and source are connected together through the device. If the main supply is turned on before the biasing supply, the main supply will be connected to ground through the transistor resulting in a current surge and possibly damaging the transistor or the power supply. Therefore, GaAs MESFET based power amplifiers usually employ a switch connected between the main supply and the drain which allows the main supply to be turned on only after the biasing supply goes negative.
The benefits of better performance of GaAs MESFETs is often offset by the additional complexity of the biasing circuitry compared to silicon transistors. In a previous two stage GaAs MESFET amplifier, the biasing circuit consisted of a converter circuit for generating a negative voltage, a voltage regulator for removing noise generated in the converter, and digital to analog converters, DACs, for converting the regulator output to the correct room-temperature biasing voltages for each amplifier stage, configured as illustrated in FIG. 1. The manufacturing of each radiotelephone containing the amplifier illustrated in FIG. 1 included the steps of measuring the required biasing voltage, and storing the digital code needed to program the DAC in a programmable read-only memory, ROM. There was no means for adjusting the bias voltage over temperature, or for adjusting the bias voltage as a means for controlling output power. The circuits which are needed for bias control including the voltage converter, the voltage regulator, the DACs, and the gain controllable driver added cost, size and weight to the radiotelephone. Furthermore, the bias voltage measurement and storage into memory added to the manufacturing cost. Finally, the lack of bias voltage control to compensate for temperature variations reduced the efficiency of power supply usage at temperature extremes. Some of these problems were addressed in more recent power amplifier design.
Another feature of a GaAs MESFET based amplifier is the isolation of the transistors from the bulk semiconductor material on the GaAs transistor chip. Thus, multiple independently operated transistors can be integrated on a single chip. These separate transistors have similar electrical properties, and in particular have gate threshold voltages which are substantially the same because they are on the same chip. Thus, the biasing circuits of FIG. 1 can be simplified by designing the amplifiers such that they reside on the same chip and can utilize a common negative bias generating circuitry, configured as shown in FIG. 2.
A further known improvement to GaAs MESFET technology employs a circuit which converts a negative input voltage into a biasing supply voltage. This biasing supply voltage tracks the negative gate threshold voltage of the transistors. A negative threshold tracking regulator is illustrated in FIG. 3. If the input negative supply voltage, Vss, is sufficiently negative, then the voltage across R3 is approximately the gate threshold voltage, Vt. In a two stage amplifier, it is often desirable to have the biasing voltage of the driver, or first power amplifier stage, set higher than the biasing voltage of the final, or second power amplifier stage. For this reason, there are two biasing output voltages, Vout1 and Vout2, connecting the gates of the first amplifier transistor and the second amplifier transistor, respectively. Since the output voltages, Vout1, minus the control voltage, Vcontrol, is equal to the voltage across R1, and the same amount of current goes through resistors R1 and R3, the output voltage, Vout1, is approximately determined by the following formula. EQU Vout1=Vt *(R1/R3)+Vcontrol (1)
and similarly, EQU Vout2=Vt *((R1+R2)/R3)+Vcontrol (2)
Since the output voltage of the circuit tracks the threshold voltage the circuit tends to suppress negative supply voltage, Vss, variations, thereby, allowing the elimination of the linear regulator circuit, the DAC and the memory of the amplifier illustrated in FIG. 2. Additionally, the manufacturing steps of measuring the threshold voltage and storing the digital code corresponding to it in the memory are also eliminated.
An amplifier employing an integrated threshold tracking negative supply generator is configured as shown in FIG. 4. Although this amplifier is greatly simplified over the original brute-force approach of FIG. 1, the amplifier chip still requires a negative supply voltage which is conventionally supplied from a separate DC to DC converter. In a typical mode of operation, R1 would be chosen to be slightly higher than R2 such that when Vcontrol is zero, the output voltage, Vout, is slightly more negative than the negative threshold voltage. When the control voltage increases, the output voltage increases by the same amount. In this way, the bias control signal Vcontrol, can be used to control output power since the amplifier output power capability is monotonically related to gate voltage over the normal operating range.
Although the foregoing descriptions of GaAs MESFET power amplifiers offer improvement over previously available power amplifiers, there is a need to maximize the efficiency that a GaAs MESFET power amplifier can offer. Specifically, there is a need for a GaAs MESFET based amplifier employing an integrated RF generated negative bias supply generator circuit where the output voltage tracks the gate threshold voltage of the GaAs FET transistors and automatically adjusts the bias voltage to the optimum value over temperature, and can be operated without current surges from the main power supply when the device is turned on, and is adjustable so that the amplifier output power can be controlled.